This invention relates to the manufacture of closures such as caps and the like, and more particularly, to a method and apparatus for molding plastic closures.
The molding of small plastic articles of even relatively simple designs often utilizes expensive and complex molding machines as exemplified by injection molding systems used for small plastic articles such as threaded closures for plastic, glass or other containers and bottles. A typical molding machine employs forming elements which are subjected to elevated temperatures and pressures. The molding machine components subjected to such conditions must be extremely rugged, necessitating the utilization of durable and expensive materials which require appropriate maintenance, repair and replacement on a regular schedule.
In one known system for the injection molding of small plastic closures, an inner mold in combination with an outer mold defines a forming volume or cavity in which a thermoplastic resin such as polypropylene or the like is injected under pressure at an elevated temperature. The various mold components must be moved relative to one another to extract or eject the molded closure from the cavity.
The art of molding plastic closures by injection, compression or other techniques is well known. Depending upon the design of the plastic closure, including the thread type or other mechanism for engaging the container, the closure may be ejected from the mold in a variety of ways. If the plastic material being molded is flexible or resilient, the closure may be stripped by a commonly known stripper ring pushing the article off of a fixed inner mold core. The molded part must be sufficiently solidified so as not to fold over onto itself during ejection but sufficiently elastic to return to essentially its original molded shape after the threads or other internal projections have been stretched over the core. If the molded material does not possess the appropriate characteristics of flexibility and rigidity for this method of ejection, the article will be damaged or may not return to its original shape and size. Moreover, a very defined or deep thread profile is inherently prone to stripping damage. Also, the closure often has other delicate or fragile features such as a tamper evident ring, which could be damaged even if an otherwise acceptable plastic is being molded.
Plastic closures can be manufactured with different features, such as continuous threads, partial or interrupted threads, or spaced hold-down lugs used commonly in child resistant safety bottles. Additionally, a closure may include a retaining rim which is used to secure a disk or insert adjacent the under surface of the upper panel of the closure. Threads, lugs or other projections from the skirt portion of the closure commonly include an undercut surface which is known to provide a more secure engagement with the threads or other mechanism on the container to which the closure is applied.
Although specialized molding systems have been proposed for the manufacture of closure caps having interrupted threads or lugs, particularly those with an undercut, such systems often include very complicated molding schemes. Commonly, a system for the manufacture of closures having interrupted threads, lugs or the like with an undercut surface includes what is usually referred to as a xe2x80x9ccollapsiblexe2x80x9d core. The collapsible core includes axially extending segments held together in an assembled configuration to form a portion of the inner mold. After the thermoplastic resin is introduced into the cavity in the mold, these segments are advanced forwardly and collapsed radially inwardly towards the axis of the mold to permit each segment to clear the threads or lugs and permit ejection of the closure without interference. Subsequently, the segments are repositioned to form the inner mold and the cycle is repeated.
However, known collapsible cores of the type described are very expensive and the construction of such molds inherently limits the industrial applicability and reliability of such molds. The complicated and often intricate interaction between the various segments and components of the molding apparatus presents significant maintenance and reliability problems. The timing and precise interaction between the various components is critical for the proper operation of known systems. Moreover, the frictional interaction between the various moving components of a mold having a collapsible core presents additional reliability and maintenance problems.
Further complicating the industrial applicability of known injection, compression and other molding systems is the difficulty of using such systems with stack molds in which a plurality of molding systems are mounted in adjacent plates. As the plates are shifted relative to one another, multiple closures are formed and ejected providing for the manufacture of thousands of closures from a single machine in a very short time frame. However, failure of one mold component necessitates service to the entire machine thereby sacrificing a significant amount of production capability while service is being performed.
Another problem with known molding systems of the type described is the difficulty in reconfiguring the molds for the production of a variety of different closure designs. Specifically, tamper evident closures are widely used to demonstrate to the final consumer that the contents of a container have not been contaminated subsequent to the time the cap was initially secured to the container. One type of tamper evident closure employs a band connected to a bottom edge of a skirt portion of the closure by a plurality of axially extending discrete, small frangible bridges or other members that are circumferentially spaced around the closure. The band includes an inside annular rib which, in use on the container, is located below a cooperating outwardly extending rib on the neck of the container. As the closure is twisted off of the container, contact between the outside rib on the container neck and the inside rib on the band breaks the previously mentioned frangible bridges, thereby separating the band from the remainder of the closure.
Providing a molded closure with a tamper evident band as described complicates the manufacture of the closure in several respects. The formation of the band and the frangible bridges connecting the band to the skirt of the closure requires that the mold used to form the closure have corresponding recesses and protrusions which complicate the removal of the closure from the mold. Naturally, the closure must be removed from the mold in a way that does not break the frangible bridges of the closure since, of course, otherwise the closure cannot be used in the intended manner. The capability for an industrial molding machine, particularly an injection molding machine, to satisfactorily mold a closure having such a tamper evident band to date has been suspect. Furthermore, the ability to re-configure a given molding machine for the production of closures without tamper evident bands to/from the production of closures with tamper evident bands often requires significant down time thereby minimizing production capacity and efficiency.
These and other disadvantages of known closure molding methods and systems have been overcome by this invention. A first presently preferred embodiment of this invention comprises an injection molding apparatus, a method of injection molding a closure and such a closure having a skirt depending downwardly from an upper panel and at least one projection extending inwardly from the skirt. The projection may be one or more intermittent or interrupted threads, lugs, stops, rims or bayonet-type engagement mechanisms for securing the closure to the upper rim of a compatible container, jar or the like.
In a presently preferred embodiment of a method for injection molding a closure, closure material such as a thermoplastic resin which is rigid, pliable, and flowable, for example styrene and polypropylene, is injected into a mold cavity formed between an outer mold and an inner mold. The outer mold includes a socket for forming the outer external surface of the closure. The inner mold includes a core and a plurality of spaced slides, each having at least one recess on an outer face thereof for forming the projection on the skirt of the closure. The slides are mounted for movement relative to the core in a direction obliquely oriented with respect to a longitudinal axis of the core to provide a collapsible inner mold.
In one embodiment, the recess on the slides includes an undercut surface to form a corresponding undercut surface on the projection, lug, thread, rim or the like of the closure. Additionally, a ring surrounds the inner mold and is mounted for movement generally parallel to the longitudinal axis of the inner mold. The injection molding method proceeds by removing the outer mold from the closure which is seated on the inner mold. In one preferred embodiment, the ring contacts the terminal edge of the skirt on the closure and the ring is mounted for movement independent from the slides on the inner mold. The closure is released from the inner mold by moving the ring in contact with the closure parallel to the longitudinal axis of the core and away from the core. The movement of the ring removes the closure off of the core and the movement of the closure translates the slides obliquely relative to the longitudinal axis of the core thereby collapsing the slides inwardly toward the longitudinal axis until the projections on the skirt of the closure disengage from the recesses on the slides. The movement of the slides to release the closure from the inner mold is advantageously a result of the interaction between the recesses and the projections on the closure and not the direct interaction between the ring and the slides. This method and the associated molding apparatus are more simplistic than known injection molding techniques. Without the closure seated on the inner mold, the forward movement of the ring would not result in the movement of the slides because the ring is separate and independently movable from the slides. The present design minimizes the number of moving parts of the molding apparatus thereby limiting the maintenance requirements for the mold.
In an alternative embodiment, the molding apparatus of this invention is used for compression molding a closure or the like. Polypropylene powder, resin or other appropriate material is introduced or placed in the cavity of the molding apparatus which is then heated to cure the molding material. The resulting closure is then ejected from the mold in a manner similar to that described with respect to the injection molding process.
A further advantage of the invention is the simplicity resulting from the reduced number of moving parts and the interchangeability of the various components. Specifically, the slides and/or the ring can be easily replaced independent from the other components of the molding apparatus. A standard ring is used for the formation of a closure without a tamper evident band; whereas, a tamper evident band forming ring is easily substituted for the standard ring when a closure having a tamper evident band is molded. Specifically, the tamper evident band forming ring includes an intaglio pattern on the ring which forms the tamper evident band on the closure. Specifically, the intaglio pattern includes relief portions for forming the tamper evident band and the frangible bridges connecting the band to the lower terminal edge of the skirt of the closure.
Additionally, a single plunger or a plurality of plungers each of which are selectively axially extendable from the core of the inner mold may be included in an alternative preferred embodiment of this invention to assist in dislodging the formed closure having a tamper evident band from the ring. Alternatively, the plunger(s) may be used to unseat the closure from the core by contacting the panel of the closure and moving the closure forwardly and thereby moving the slides obliquely and inwardly towards the longitudinal axis of the inner mold to release the recesses on the slides from the projections on the closure. The plunger may be used to advance the closure in this manner in place of and/or in addition to the movement of the ring surrounding the closure. The capability of simply swapping the tamper evident forming band ring with the standard ring without the requirement for making an entirely new molding apparatus is a key advantage for the industrial applicability of this invention.
It should be readily appreciated by those of ordinary skill in the art that a molding apparatus according to this invention may be employed in a stack mold configuration in which each of the molding components are seated within respective plates and the plates are longitudinally movable relative to one another during the automated molding process. A stack mold of the type described is shown and disclosed in U.S. Pat. No. 4,019,711 which is hereby incorporated by reference; although, the particular mold components and molding method employed in the stack mold configuration as shown in that patent are distinctly different from those of this invention.
Therefore, as a result of this invention, an improved method and mold for injection, compression or other molding of a plastic closure having undercut projections such as threads, lugs, stops or the like is provided having increased simplicity, interchangeability and applicability for industrial molding manufacturing applications.